Method and production of a sensor

ABSTRACT

The invention relates to a method for producing a sensor ( 1 ), wherein a carrier chip ( 2 ) is produced. Said chip is provided with a sensor structure ( 3 ) comprising an active sensor surface ( 4 ). A material ( 9 ) capable of flowing is applied onto carrier chips ( 2 ) in such a way that the sensor structure ( 3 ) has a thinner layer thickness on said active sensor surface ( 4 ) than on the area of the carrier chip ( 2 ) which borders on the active sensor surface ( 4 ). The material ( 9 ) which is capable of flowing is hardened thereafter. The hardened material ( 9 ) is subsequently removed by chemical means from the surface which faces said carrier chip ( 2 ) until the active sensor surface of the sensor structure is layed bare.

BACKGROUND OF THE INVENTION

The present invention relates to the field of semiconductor sensors, andin particular to a semiconductor sensor device that includes a carrierchip comprising a sensor structure with an active sensor surface,wherein a flowable material is applied to and bounds the active sensorsurface region of the carrier chip, and the flowable material is thensolidified.

As known, for manufacturing a semiconductor sensor device, a block isfirst placed on the active sensor surface before flowable material isapplied to the sensor chip. The outer contour of the block is matched tothe outer contour of the sensor surface, so that, when the block is inits operational position, it covers only the sensor surface, while theregions surrounding the sensor surface are not covered by the block. Theform of the surface structure of the block area that contacts the activesensor surface is matched to that of the active sensor surface, so theblock seated on the sensor surface seals it against the flowablematerial. This material is then applied to the surface of the sensorchip in the area surrounding the block, and is then solidified. Forexample, the flowable material can be a plastic compound that cures orrigidifies after being applied to the sensor chip. The active sensorsurface is then freely accessible for contact with a medium beinginvestigated, while the surface regions laterally surrounding the activesensor surface are sealed by the plastic compound.

However, this method has a disadvantage that the block must bepositioned on the sensor surface very precisely, to prevent the flowablematerial from penetrating between the block and the sensor surface. Thatis, the block must be precisely positioned to prevent the sensor surfacefrom being covered with the flowable material. A further disadvantage isthat the carrier chip is damaged when the block is placed on the sensorsurface, and thus the device is rendered useless or at least defective.Another disadvantage of this method is that the production of the blockand especially of the surface structure of that component which iscomplementary to the surface structure of the sensor surface when theblock is in its operating position is still comparatively expensive.Finally, this known method is also unsuited for automation, and thus theeconomical mass production of sensors.

Therefore, there is a need for a technique of providing a protectivecovering over the sensor surface, the application of which does notdamage the sensor.

SUMMARY OF THE INVENTION

Briefly, according to an aspect of the invention, a method of enclosinga semiconductor die operably attached to a lead frame, includes placingthe die operably positioned on the lead frame into an injection moldingtool and injection molding a flowable material into the tool to cover atop surface of the semiconductor die. The flowable material covering anactive sensor surface has a thickness of flowable material that is lessthick than the flowable material covering regions adjacent to the activesensor surface, to provide a non-uniformly coated die assembly. Thenon-uniformly coated die assembly is removed from the injection moldingtool, and hardened flowable material is etched away until the activesensor surface is exposed, to provide an etched coated die assembly. Theetched coated die assembly is again placed into the injection moldingtool and a flowable additive material is injected into the tool to coverthe exposed active sensor surface.

The flowable material is applied less thick on the active sensor surfaceof the sensor structure than on that region of the carrier chip thatsurrounds the active sensor surface. After the flowable material hassolidified, it is chemically ablated over its entire surface that facesaway from the carrier chip until the active sensor surface of the sensorstructure is exposed.

The flowable material is applied to the region of the carrier chip thatsurrounds the sensor surface, and also on the sensor surface itself,although the sensor surface actually should ideally be kept free of theflowable material. In the region of the active sensor surface, theflowable material is applied less thick than in the laterally adjoiningregions. After the flowable material has solidified, it is brought incontact with a medium that makes a chemical bond with the solidifiedmaterial, such that the solidified material is ablated. This contact ismade both in the region of the active sensor surface and in thelaterally adjoining regions of the flowable material, at that surfacewhich faces away from the carrier chip. The chemical ablation process isstopped when the solidified material in the region of the active sensorsurface has been completely ablated and thus the sensor surface situatedbeneath this has been exposed. Since the flowable material has beenapplied thicker in the regions that laterally adjoin the sensor surfacethan in the region of the sensor surface itself, the regions laterallyadjoining the sensor surface continue to be covered by the solidifiedflowable material after the sensor surface has been exposed. Thechemical medium for ablating the solidified material is chosen so thatit does not chemically attack the material that constitutes at leastthat part of the sensor structure which bears the sensor surface. Theinventive method thus makes it possible, in simple fashion, to apply, onthe sensor structure, a lateral boundary to the sensor surface, suchthat the sensor surface remains free of this boundary. Damage to thesensor structure is reliably avoided. The flowable material cansolidify, in well-known fashion, by curing, solidifying, and/or thevaporization of a solvent contained in the flowable material.

The flowable material is ablated by etching after it has solidified, forexample by plasma etching. The method can then be especially wellintegrated into a semiconductor production process, wherein the carrierchip and/or the sensor structure situated thereon include asemiconductor material.

The flowable material can be spray-applied to the carrier chip in aninjection molding tool. The injection molding tool may have a block-likeprotrusion, which, in its operational position, faces and covers theactive sensor surface, so that its boundary is flush with that of theactive sensor surface. The carrier chip that bears the sensor structurecan then very simply be encapsulated with the flowable material designedas a transfer molding. A transfer molding such as, for example athermoplastic, which is used to encapsulate conventional electroniccomponents can be used here. This method is especially suited for theindustrial mass production of large numbers of sensors.

In one embodiment, a flowable additive material that covers the sensorsurface is applied at least to the exposed surface of the sensorstructure, and is then solidified. The additive material contacts theactive sensor surface, while the remaining regions of the sensorstructure are kept at a distance from the additive material through thesolidified material that forms the boundary of the sensor surface. Inthis way, it is possible to apply an additive material to the sensorstructure that may come in contact only with the active sensor surface,but not with the remaining regions of the sensor structure. The additivematerial can be especially a material that is necessary for the sensorto function, such as for example an ion-permeable membrane or anion-selective sensor, a transparent layer for an optical sensor, anoptical filter, an optical lens, or another material that differs fromthe solidified material which forms the boundary.

The flowable additive material may be sprayed on the sensor in theinjection molding tool. The additive material then fills the chemicallyablated regions of the solidified material, which has been applied tothe sensor structure or to the carrier chip as a boundary for the sensorsurface. Advantageously, the same injection molding tool can be used tospray on the additive material as was used to spray on the materialforming the boundary of the sensor surface. As a result, an expensive,second injection molding tool is not required.

One embodiment of the invention provides that the carrier chip isdisposed on a frame when the flowable material and, where applicable,the additive material is being applied, and that openings to receiveand/or pass component regions of the frame are situated in the innercavity of the injection molding tool, and that, in the operatingposition, the bounding edges of the openings contact these componentregions so as to create a seal. These component regions of the framethen remain free of the flowable material and, where applicable, also ofthe additive material. For example, they can be equipped with externalelectrical contacts for connecting the sensor to an electrical orelectronic circuit. The connection contacts can be connected to thesensor structure through bond wires which contact, on the one hand, theframe and, on the other hand, the carrier chip and/or the sensorstructure.

The flowable material may be a photosensitive material. The material isthen ablated by regions, by a masking and development process. Thematerial is ablated such that the material which remains on the activesensor surface of the sensor structure forms a thinner layer than itdoes in that region of the carrier chip which surrounds the activesensor surface. The material is chemically ablated over its entiresurface, until the active sensor surface of the sensor structure hasbeen exposed.

After the flowable, photosensitive material has been applied to thecarrier chip, it is conditioned if necessary, for example by heat.During the subsequent masking process, the photosensitive material isexposed by regions, for example by projecting optical radiation througha photo mask onto the photosensitive material. When a photopositivematerial is used, the photosensitive material situated on the activesensor surface is exposed, while the remaining photosensitive materialremains unexposed. Therefore, when a photonegative material is used, thephotosensitive material situated on that surface region of the carrierchip that bounds the active sensor surface is exposed, while thephotosensitive material situated on the active sensor surface remainsunexposed. In the following development process, the material situatedon the active sensor surface is then ablated more strongly upon contactwith the developer than the remaining material, which is essentiallyresistant to the developer. The exposure and development process can beguided such that the material situated on the active sensor surfaceafter development is only a very thin layer. This practically obviatesthe risk that, during the subsequent chemical ablation of the remainingmaterial over the entire area, undercuts or under-etchings will form inthe material regions that adjoin the edge of the active sensor surface.This is especially advantageous if the remaining material is ablated byisotropic etching.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a partial cross section through an injection molding tool,in which a semiconductor chip, which has an active sensor surface andwhich is situated on a frame is encapsulated by a flowable materialthrough injection molding;

FIG. 2 shows a partial cross section of the semiconductor chip demoldedfrom the injection molding tool, when the sprayed-on and solidifiedmaterial is ablated;

FIG. 3 shows a partial cross section through the arrangement of FIG. 2,after the active sensor surface of the semiconductor chip has beenexposed; and

FIG. 4 shows a partial cross section through the sensor shown in FIG. 3,after it has been encapsulated with an additive material by injectionmolding.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a partial cross section through an injection molding tool,in which a semiconductor sensor chip 1 that includes an active sensorsurface is situated on a frame and is encapsulated. The sensor 1includes a carrier chip 2, which is designed as a semiconductor chip,and which has a sensor structure 3 with an active sensor surface 4. Thecarrier chip 2 is disposed on a frame 5, together with other carrierchips. The carrier chip 2 adheres to the frame 5 through an adhesivelayer 6, which is located between the carrier chip 2 and the frame 5.The carrier chip 2, situated on the frame 5, is placed into the innercavity of an injection molding tool, which has two molding parts 7 a, 7b, that can move relative to one another and that can be brought into anopen and closed position.

The injection molding tool has openings (not shown in the drawing) whichare situated in the separation plane of the mold parts 7 a, 7 b. Whenthe mold parts 7 a, 7 b are in the closed position, these openings takeup component regions of the frame 5, which is inserted in the innercavity of the injection molding tool. The shape of the openings ismatched to the shape of the component regions of the frame 5 inserted inthem, so that, when the mold parts 7 a, 7 b are in the closed position,the bounding edges of the openings always contact the component regionsof the frame 5, which are inserted into the openings, in such a way asto create a seal. At the openings, the frame is interlocked with themold parts 7 a, 7 b, which are in their closed position.

FIG. 1 shows that the oppositely facing, flat sides of the arrangementformed by the carrier chip 2, the frame 5, and the adhesive layer 6always are at a distance from the inside walls of the mold parts 7 a, 7b, which are situated in their closed position. Referring still to FIG.1, the mold part 7 a of the injection molding tool has a block-likeprotrusion 8. When the injection molding tool is in its closed position,the block-like protrusion 8 faces the active sensor surface 4 of thecarrier chip 2, which is inserted into the inner cavity of the injectionmolding tool. The block-like protrusion 8 then covers the sensor surface4, such that the bounding edge of the block-like protrusion 8 coincideswith that of the active sensor surface 4. FIG. 1 shows that, in theregion of the sensor surface 4, the carrier chip 2 has a lesser distancefrom the inside wall of the mold part 7 a, in a direction orthogonal tothe extensive plane of the carrier chip 2, than in the regions of thecarrier chip 2, which laterally adjoin the sensor surface 4.

After the injection molding tool has been closed, the carrier chip 2located within the tool is encapsulated by injection molding in aflowable material 9, such as for example a plastic. The flowablematerial 9 is introduced into the inner cavity of the injection moldingtool through inlet ducts, which are not shown in the drawing, underpressure and possibly under the action of heat. The flowable material 9fills the free spaces situated between the arrangement formed by thecarrier chip 2, the frame 5, and the adhesive layer 6, and the walls ofthe mold parts 7 a, 7 b. FIG. 1 clearly shows that the flowable materialis applied less thick on the active sensor surface 4 of the sensorstructure 3 than on the regions of the carrier chip 2, which laterallybound the active sensor surface 4. After the carrier chip has beenencapsulated by injection molding, the material 9 is solidified andrigidified by cooling. The encapsulated carrier chips 2, together withthe frame 5, are then demolded (i.e., removed) from the injectionmolding tool.

The surface of the material 9 that faces away from the carrier chip 2 isthen ablated by etching, for example by wet etching or by plasmaetching. In FIG. 2, the ablation process is symbolically represented byarrows. The etching process is stopped when the material 9 has beencompletely ablated from the carrier chip 2 in the region of the activesensor surface 4, and the sensor surface 4 situated beneath this isexposed. The etching technique employed is chosen so that the materialof the sensor structure 3 is chemically stable against the etchingtechnique, and thus the sensor structure 3 is not altered by contactwith the etching material. As FIG. 3 illustrates the regions of thecarrier chip 2, which laterally adjoin the sensor surface 4, andcontinue to be covered by the material 9 after the sensor surface 4 hasbeen exposed.

After the sensor surface 4 is exposed, the frame 5 together with thecarrier chips 2 adhering thereto through the adhesive layer 6, is againinserted into the injection molding tool, and the mold parts 7 a, 7 bare closed. The above-mentioned component regions of the frame 5 nowengage the openings of the injection molding tool assigned to them, suchthat the frame 5 is positioned in the inside cavity of the injectionmolding tool, in the same way as when it is being encapsulated by theflowable material 9. At locations where the material 9 has been etchedaway at the encapsulation of the carrier chip 2, a free space remainsbetween the carrier chip 2, encapsulated by the material 9, and therespectively adjoining inside wall of the mold parts 7 a, 7 b. Aflowable additive material 10 is injected into this free space throughthe inlet ducts of the mold parts 7 a, 7 b to fill the free space. Theadditive material 10 then solidifies, for example by rigidifying and/orcuring. In the embodiment illustrated in FIG. 4, the additive material10 is optically transparent, and forms a window that is transparent tothe visible radiation that is to be detected. The radiation to bedetected can pass through this window to the light-sensitive sensorsurface 4 of the sensor structure 3.

By the method for producing the sensor 1, a carrier chip 2 is thusproduced, which has the sensor structure 3 with an active sensor surface4. A flowable material 9 is applied to the carrier chips 2, in such away that it forms a thinner layer on the active sensor surface 4 of thesensor structure 3 than on the regions of the carrier chip 2, whichlaterally adjoin the active sensor surface 4. The flowable material 9 isthen solidified. The surface of the solidified material 9 that facesaway from the carrier chip 2 is then chemically ablated, until theactive sensor surface of the sensor structure is exposed.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. A method for producing a sensor, such that a carrier chip isproduced, which has a sensor structure with an active sensor surface, aflowable material is applied to that surface region of the carrier chip,which bounds the active sensor surface, and this material is thensolidified, wherein the flowable material is applied less thick on theactive sensor surface of the sensor structure than on the region of thecarrier chip, which surrounds the active sensor surface, and that, afterthe flowable material has solidified, it is chemically ablated over itsentire surface which faces away from the carrier chip, until the activesensor surface of the sensor structure is exposed.
 2. The method ofclaim 1, wherein after the flowable material has solidified, it isablated by etching.
 3. The method of claim 2, wherein the carrier chipis encapsulated by the flowable material in an injection molding tool,and that the injection molding tool has a block-like protrusion, which,in its operating position, faces the active sensor surface, such thatits bounding edge is flush with that of the active sensor surface. 4.The method of claim 3, wherein a flowable additive material is appliedat least on the exposed sensor surface of the sensor structure to coverthe sensor surface, and is then solidified.
 5. The method of claim 3,wherein the flowable additive material is sprayed on the sensor in theinjection molding tool.
 6. The method of claim 5, wherein the carrierchip is disposed on a frame when the flowable material and, whereapplicable, the additive material is being applied, and that openings toreceive and/or pass component regions of the frame are situated in theinner cavity of the injection molding tool, and that, in the operatingposition, the bounding edges of the openings contact these componentregions so as to create a seal.
 7. The method of claim 5, wherein aphotosensitive material is applied, as the flowable material, to theactive sensor surface and the surrounding surface region of the carrierchip, and that the material is then ablated by regions, by a masking anddevelopment process, in such a way that the material which remains onthe active sensor surface of the sensor structure forms a thinner layerthan it does in the region of the carrier chip, which surrounds theactive sensor surface, and that the material is then chemically ablatedover its entire surface, until the active sensor surface of the sensorstructure has been exposed.
 8. A method of enclosing a semiconductor dieattached to a lead frame, said method comprising: placing the dieoperably positioned on the lead frame into an injection molding tool;injection molding a flowable material into the tool to cover a topsurface of the semiconductor die, wherein the flowable material coveringan active sensor surface has a thickness of flowable material that isless thick than the flowable material covering regions adjacent to theactive sensor surface, to provide a non-uniformly coated die assembly;removing the non-uniformly coated die assembly from the injectionmolding tool, and etching away hardened flowable material until theactive sensor surface is exposed, to provide an etched coated dieassembly; and placing the etched coated die assembly into the injectionmolding tool and injecting a flowable additive material into the tool tocover the exposed active sensor surface.
 9. The method of claim 8,wherein the flowable additive material is optically transparent.
 10. Themethod of claim 9, wherein the flowable material is a plastic material.11. The method of claim 10, wherein said step of etching comprisesplasma etching.
 12. A method of enclosing a sensor having an activesensor surface located on a top surface of a semiconductor die attachedto a lead frame, said method comprising: applying a flowable material tocover a top surface of the semiconductor die using a first tool, whereinthe flowable material covering the active sensor surface has a thicknessof flowable material that is less thick than the flowable materialcovering regions adjacent to the active sensor surface, to provide anon-uniformly coated die assembly that has a relatively planar topsurface of flowable material; ablating hardened flowable material untilthe active sensor surface is exposed, to provide an etched coated dieassembly; and applying flowable additive material to cover the exposedactive sensor surface using the first tool.
 13. The method of claim 12,wherein the flowable additive material is optically transparent.
 14. Themethod of claim 12, wherein prior to said step of applying flowablematerial, placing the die operably positioned on the lead frame into thefirst tool, which is an injection molding tool.
 15. The method of claim12, wherein said step of applying flowable additive material comprisesplacing the etched coated die assembly into the injection molding tool,and then covering the exposed active sensor surface with the flowableadditive material.
 16. The method of claim 15, wherein the flowablematerial is a plastic material.
 17. The method of claim 12, wherein saidstep of ablating comprises chemically etching away the hardenedmaterial.
 18. The method of claim 17, wherein said step of chemicallyetching comprises plasma etching.